This is a .20 caliber case neck mandrel for the RCBS hand case neck turner.

The RCBS Hand Case Neck Turner is designed to "peel" the brass from the outside of your case necks to a consistent thickness. Most commercially available brass has variances in the case neck thickness, which leads to case neck concentricity and bullet runout issues. Truing up the case neck produces more consistent and accurate hand loaded ammunition. The cutter portion uses caliber specific mandrels to guide the case neck.